1. Field of the Invention
The invention relates to lithium niobate devices. More particularly, the invention relates to dissipating accumulated charge on the dielectric surfaces of lithium niobate devices.
2. Description of the Related Art
Electrooptic devices employing materials such as lithium niobate often are used as modulators, e.g., within fiber optic communication systems, for signal processing applications, and as sensors. These modulators include, e.g., optical intensity modulators, switches, phase or frequency shifters, polarization transformers and wavelength filters.
An operational shortcoming of these devices is temperature and pressure dependence within the devices. More specifically, these lithium niobate devices typically suffer from charge accumulation brought about by the self biasing effects resulting from, e.g., pyroelectric and/or piezoelectric effects within the devices. Conventional attempts to dissipate the accumulated charge via the deposition of a charge dissipation layer have not yielded satisfactory results.
For example, in U.S. Pat. No. 5,388,170, which is co-owned with this application and is assigned to the assignee of this application, a charge dissipation layer formed on the bottom or back side of the device (i.e., the non-electrode side of the device). The charge dissipation layer is made of a "metal" and/or a "semiconductor material". Also, disclosed in U.S. Pat. No. 5,388,170 is a charge dissipation layer, made of indium tin-oxide (ITO) or polycrystaline silicon, formed on the electrode side of the device. However, such configuration is difficult to fabricate consistently and it is uncertain whether the claimed resistivity values can be achieved (see, H. Nagata, et al., "Comments on fabrication parameters for reducing thermal drift on LiNbO.sub.3 optical modulators", Society of Photo-Optical Instrumentation Engineers, January 1997, pp. 283-286).
Also, Fujitsu apparently has disclosed the use of a polysilicon charge dissipation layer (see, Japanese Patent Application No. Sho 621987!-173428). However, such high reported resistivity values are not realistic for polysilicon and result from some partial oxidation of the film. These partially oxidized polysilicon films are achieved, e.g., by elevated temperature partial oxidation, which is a poorly controlled process. Thus, at times the films will have the proper resistivity but at other times the resistivity of the films will be unacceptable. Accordingly, the use of "as deposited" oxygen or nitrogen contaminated films and their reproducible resistivities has been desired but heretofore has not provided satisfactory results.
It would be desirable to have available a method and apparatus for dissipating the accumulated charge on lithium niobate devices such as lithium niobate modulators.